It has been known for some time that tissue ablation can be enhanced through the use of infrared wavelengths that more closely match absorption peaks of water, the major constituent in biological tissue. Accordingly, the industry has recently been exploring the use of erbium-doped gain media for medical applications. Erbium-doped YAG crystals will generate an output wavelength of about 2.9 micrometers (.mu.m) which is matched to a prominent absorption peak in water. Er:YAG radiation can be particularly useful in cosmetic or aesthetic laser surgical applications such as skin resurfacing, as the high absorption coefficient for the radiation in water limits penetration depth of the radiation essentially to the skin.
In skin resurfacing, skin is removed by photoablation using a high energy pulse, for example, on the order of about 1 Joule (J). The pulse has a relatively short duration, for example about 200 to 500 microseconds (.mu.s). Energy is applied in a beam having a diameter of about 2 to 3 millimeters (mm) at pulse rates between about 1 to 15 Hertz (Hz). The short pulse duration provides that no appreciable heat is generated, which limits collateral damage. The short penetration depth provides that healing (new skin growth) can be complete in about two weeks or less.
The short penetration depth of Er:YAG radiation is also potentially attractive for cosmetic procedures which require skin incision in sensitive areas. An example of such a procedure is blepharoplasty, for rejuvenating the appearance of aging eyelids. This procedure requires an incision to be made very close to an eyelid, consequently a very precise cut is required. To make such a precise cut, an Er:YAG laser beam is preferably focussed to a much smaller spot, for example, about 200 to 300 .mu.m in diameter. Focussing to a small spot provides not only the required precision, but provides that less energy per pulse is required to reach an ablation threshold intensity. These low energy pulses are preferably delivered at a relatively rapid repetition rate, for example, preferably at least 50 Hz, and more preferably about 100 Hz or even greater.
Erbium laser systems operable at up to about 30 Hz are commercially available. Attempts to increase the repetition rate above this level, however, have met with problems in obtaining adequate output power as well as problems with the thermal loading and accompanying thermal-lensing of the laser gain-medium (laser-rod). Thermal-lensing results from heating of the rod by the portion of absorbed pump-light which is not extracted as laser energy and can lead to instabilities of the laser resonator, and reduce the upper limit of pump-power.
One scheme for providing repetition rates of about 100 Hz in a single resonator with reduced instability is described in U.S. Pat. No. 5,644,585. This scheme, however, involves decreasing output power by increasing the reflectivity (decreasing transmission) of a resonant cavity. The higher reflectance of the output coupling mirror decreases the flashlamp power required to provide lasing, which in turn reduces thermal loading on the laser-rod. For operating at pulse rates higher than 200 Hz, without increasing thermal loading problems, the outputs of two or more resonant cavities are interleaved such that pulses from one resonant cavity are delivered in an interval between pulses from another resonant cavity. This of course leads to a system of significantly increased cost and complexity compared with single resonant cavity system.
There remains a need for a pulsed Er:Yag laser capable of providing a high quality beam of relatively low energy pulses at pulse repetition rates up to about 200 Hz or greater for providing smooth, precise incisions, but which can also provide controllable higher energy pulses at lower repetition rates down to about 1 Hz. Such a laser could be used for a wide range of surgical procedures, including large area operations such as skin resurfacing which typically require high pulse energy. This could significantly reduce the capital and maintenance cost of laser equipment for a surgeon, which in turn could lead to reduced operation costs and increased access to laser surgical procedures for patients.